1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor thin film adapted to be used as a Hall-effect device which is used to detect rotations, displacements, and other motions.
2. Description of the Related Art
Hall-effect devices using semiconductors have several advantages. For example, they have excellent frequency characteristics, are capable of noncontacting-type detection, and are insusceptible to noise. They have been used principally as noncontacting-type rotational number-detecting sensors and have found wide application. Among them, a Hall-effect device using indium antimonide (InSb) which is a semiconductor having the greatest electron mobility produces a large output signal. Moreover, a wide gap can be secured between this Hall-effect device and a sample to be investigated. Hence, this device is most suited for a rotational number-detecting and displacement-detecting sensor. Hall-effect devices using InSb include magneto-resistors and Hall generators. The prior art techniques are described below in further detail, using the magneto-resistors.
Conventional InSb magneto-resistors are classified into the bulk type and the thin-film type, according to the fabrication process. The bulk type is fabricated by bonding a single bulk crystal to the top .surface of a support substrate with adhesive, polishing the crystal into a thin leaf, and machining or processing the leaf. In this structure, the electron mobility has the greatest values of 5 to 8 m.sup.2 /V s at room temperature because of the use of a single bulk crystal. Also, this bulk-type structure produces a large output signal. However, it is not easy to handle this device because the electron mobility depends heavily on temperature. Another problem is that the InSb thin film cracks due to a difference in coefficient of thermal expansion between the adhesive layer and the InSb at elevated temperatures. Therefore, the operating temperature of the device is restricted to the range from approximately -20.degree. C. to +80.degree. C., In high-temperature applications, for example in automobile applications where the used temperature range is between -50.degree. C and +150.degree. C., the device lacks reliability and hence is not used.
On the other hand, the latter thin-film type is fabricated by a vacuum process such as vacuum evaporation techniques. In particular, a thin film of InSb is formed on a substrate by a vacuum process and then the film is machined or processed. The device fabricated by this method is inferior in electron mobility to a single bulk crystal because of grain boundaries and dislocations existing inside the film but the temperature-dependence is milder and so the thin-film type device can be easily handled. Furthermore, there is a possibility that reliability is secured in high-temperature applications, since the thin film of. InSb is directly formed on the substrate. In addition, the thin-film type device can be made thinner than the bulk type. This makes it easy to increase the resistance of the device. In consequence, low electric power consumption and miniaturization can be accomplished.
In the thin-film type, however, the used substrate plays an important role. As an example, where an In Sn film is formed on a substrate whose surface is amorphous such as a glass substrate, the obtained film is polycrystalline. The electron mobility is 2 to 3 m.sup.2 /V s at best, and the output signal from this device is small. Fukunaka and others have obtained an electron mobility comparable to that of a single crystal by the use of a substrate of cleaved mica (Technical Report of Toyo Tsushin-ki, No. 40, (1987)). In this method, however, the adhesiveness between an InSb thin film and mica substrate is bad. Thus, it is necessary to transfer the InSb thin film to another support substrate via an adhesive layer. For this reason, the usable temperature range is restricted to a range similar to the range for the bulk type. Other known techniques utilize molecular beam epitaxy to epitaxially grow an InSb film on a substrate made of CdTe, sapphire, BaF.sub.2, GaAs, or other material. Unfortunately, this substrate is very expensive.
Chyi and others produced a thin film of InSb having an electron mobility of 3.9 m.sup.2 /V s on a substrate of a relatively cheap, silicon (Si) single crystal by molecular beam epitaxy (J.-I. Chyi et al., Appl. Phys. Lett., 54, 11 (1989)). However, this method needs a manufacturing step where the Si surface is maintained above 900.degree. C. under an ultrahigh vacuum (normally below 10.sup.-7 Pa) to remove the oxide film on the surface of the Si. It is not easy to use this step in the manufacturing process. In this way, with the thin film type, any method of forming a thin film of InSb having a high electron mobility directly on a substrate at low cost is not available and so the thin-film type has not enjoyed wide acceptance.